At lower rpm we find the spring to oscillate very violently but as we increase the rpm the vibrations were much lesser.
It depends from the resonant frequency of the system. The behaviour it follows can be explained on the basis of the transmissibility of vibration. Up the resonant frequency of the system the amplitude of vibration decreses of about 40 dB/decade, as a function of frequency (i.e. increasing rpm). The value of frequency (rpm) at which the oscillation reaches its maximum amplitude, that is the resonant frequency of the system.
We see a mass and a spring: a mass-spring system. These systems do have an eigen frequency: if excited by a pulse, the mass will move with this frequency.
If an external force is applied -as in the video- the mass will move easily and therefor with high amplitude if the frequency of the external force equels the eigen frequency.
N.B. Shouldn't you have read this in your textbook before starting the experiment?
I am adding the following notes to Evert and Alessandro's answers above;
Regarding resonant frequency matching; I recommend you to extract natural frequencies of the system. Your external excitations triggered from external forces need not to match your natural frequencies. Generally speaking; if a system vibrates at lower frequencies, it means that this system doesn't have a robust and a good design. Therefore, after extracting the natural frequencies of the system, it is highly recommended to shift these vibrations at higher frequencies by controlling the ration between the mass and the stiffness of the system, and of course I totally agree with the answers above as well, hopefully this is helpful for you!
Best luck
Mohamed Alzoubi
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